High Efficiency Indirect Air Economizer-based Cooling for Data Centers

Overview

Wendy Torell's paper argues that an indirect air economizer can keep outside contaminants and humidity swings out of the IT space while matching the energy performance of direct fresh-air cooling. Four design principles carry the argument: make economizer the primary operating mode, use mechanical cooling only as trim or backup, widen the IT temperature window, and build the system modular, pre-engineered and with integrated controls.

The numbers show the effect. Air-to-air alone gives 2,419 hours of full economizer (28% of the year) at 21.1 C IT inlet and 4,070 hours (47%) at 27 C. With evaporative assist, this rises to 4,394 hours (50%) at 21.1 C and 6,289 hours (72%) at 27 C. A traditional chilled-water plant runs economizer about 25% of the year, against more than 50% for the indirect evaporative design. The paper claims these principles cut cooling energy by about 50% versus traditional designs.

Footprint and water are compared too. Typical data centres lose 10 to 20% of white space to physical infrastructure; indirect modules placed outside use about 30 m2 per 100 kW of IT, roughly 5% of computer-room space. On water, cooling towers use about 40 gpm per 1,000 tons (151.4 L/min) every hour of the year, while the indirect evaporative system uses water only when evaporative assist runs. The text is a design argument, and the energy-saving claim rests on the author's design principles; the hour counts are given for one climate and specific IT inlet temperatures. For a different site, economizer hours need to be recalculated.

Key points

1. The design principles are claimed to cut cooling energy by about 50% versus traditional designs.
2. Air-to-air only: 2,419 h (28% of the year) of full economizer at 21.1 C IT inlet; 4,070 h (47%) at 27 C.
3. With evaporative assist: 4,394 h (50%) at 21.1 C; 6,289 h (72%) at 27 C.
4. Typical data centres lose 10 to 20% of white space to physical infrastructure.
5. Indirect modules placed outside use about 30 m2 per 100 kW of IT, about 5% of computer-room space.
6. A traditional chilled-water plant runs economizer about 25% of the year versus more than 50% for the indirect evaporative design.
7. Cooling towers use about 40 gpm per 1,000 tons (151.4 L/min) every hour; indirect evaporative cooling uses water only when evaporative assist runs.

Using it in design

• Add the indirect evaporative cooler (IEC) as a plant type, with parameters for approach temperature, wet-bulb effectiveness, trim DX capacity fraction and water use only during evaporative hours.
• In the economizer-hours calculator, give a slider for IT inlet setpoint from 21 to 27 C and show the change in hours live.
• At sites where direct outside air is ruled out by air quality or humidity, compare the indirect module against the direct fresh-air option on the same energy metric.
• Reserve the module footprint (about 30 m2 per 100 kW of IT) as outdoor area on the site plan.

Who should read it

Designers and investors who want low-energy cooling but cannot accept direct fresh air because of air quality or humidity.

Related standards

• ASHRAE TC9.9 Thermal Guidelines
• ASHRAE 90.4

Publication details

Authors

Wendy Torell

Publisher

Schneider Electric

Year

2017

Document number

WP 136

Document language

English

Source type

Manufacturer

Access

Free to read

Licence

Copyright retained; citation only — the publisher’s terms apply to the original document

Citation